1. Field of the Invention
The present invention relates to medical devices and, more specifically without limitation, to implanted medical devices.
2. Description of the Related Art
The advent of microprocessors has stimulated proliferation of implantable medical devices for monitoring and treatment of cardio-vascular and neurological diseases; examples of such devices are cardiac pacemakers, implantable cardioverter defibrillators (ICDs), nerve stimulators, brain stimulators for Parkinson's disease and tremor therapy, pain therapy stimulators, spinal cord stimulators, cochlear stimulators, prostheses, infusion pumps, and devices for warning of seizures, syncope or falls. Verification of the functional status of these devices can be obtained through user-initiated interrogation or in the case of battery status, automatically. Despite their technical sophistication, unintentional or accidental shut off, or non-user programmed changes in recording/detection or stimulation parameters (e.g., caused by radiation, electromagnetic or electrostatic discharges, and power line interference) remain as serious, unaddressed problems. Of all these potential problems, inadvertent or accidental shut-off of the device or of its output is the most difficult to address because of its unpredictability and also the most serious because of the consequences these failures may have on subjects with potentially fatal cardiac or neurological diseases; this is particularly true if their manifestations are intermittent and unpredictable, as is the case with cardiac arrhythmias and seizures. Envision, for instance, the outcome of inadvertent shut-off caused by, for example, electromagnetic interference of an implantable defibrillator, demand pacemaker, or of a seizure blockage device. Consideration of any of these possible scenarios leads to the inevitable conclusion that the ability to automatically and without delay detect and warn the user about device failure is potentially a life-saving feature, regardless of the mode of operation (continuous or intermittent) of the device. The inability of present state-of-the-art devices to automatically detect failure is particularly egregious, especially for devices programmed to operate only in response to specific events, such as demand pacemakers, ICDs, and certain neuro-stimulators. While causes of accidental shut-off of a device's power or of its output have been identified and patients are instructed about potential hazards and safety issues regarding exposure to sources of electromagnetic energy, it is very difficult, if patients are to lead a normal life, to avoid these sources and the risk of accidental interference with the normal operation of the device. To summarize, in spite of the highly advanced features programmed or built into state-of-the-art medical devices, none have the capability to immediately and automatically detect and communicate the occurrence of failures or of improper functioning to the user.
A schematic representation of a prior art implantable medical device is depicted in FIG. 1. When turned on, the prior art devices start with some default parameters, which are then modified by physicians or other qualified personnel to suit a particular mode of operation. These devices then generally operate in this mode for several weeks or sometimes even years. Basic automated checks for low battery, lead integrity, and memory accessibility exist for many of these devices. A schematic representation of a typical failure of a prior art implantable medical device, which can occur at any time in a prior art device, is depicted in FIG. 2. Failures are most dangerous and most likely to occur when the device is conducting an unsupervised operation, which is generally the predominant operating mode of such devices. Once failures or faults occur, they may remain undetected until the next scheduled user-initiated interrogation or the occurrence of a serious or fatal adverse event, whichever occurs first.
The prior art in the area of device status diagnostics provides means to inform the user of impending delivery of electrical currents, changes in programmed parameters (identifiable only during interrogation) and low battery by using audible or visible alarms, low-voltage “tickler” stimulation, or vibration, but no means to detect accidental device or output shut-off and report them as soon as they occur. For example, U.S. Pat. Nos. 4,024,875, 4,488,555, 4,345,603, 4,445,512, and 4,102,346 disclose means for detecting and warning about low battery or low voltage. Several other patents, namely U.S. Pat. Nos. 6,067,473, 6,450,172, 5,891,180, and 6,247,474, mention using audible signals to deliver messages related to low battery, impending delivery of therapy, and acknowledgement of programming to user, but none of these patents provides a method or program for automatically detecting and warning the user, without delay, that the monitoring/therapeutic medical device has failed or that the output has been shut-off. U.S. Pat. No. 5,197,468 discloses a different approach via attaching a protective device to reduce the effects of RF or electrostatic energy on a medical device which, although useful, does not provide patients with the necessary protection and warning in case of failure. There are devices on the market (e.g., Marquis® Family of ICDs from Medtronic, Inc.) that have self-monitoring patient alert systems (Expanded Patient Alert™) for certain kinds of problems, such as improper programming, out of range parameters, low battery and faulty leads. U.S. Pat. No. 3,805,796 discloses means, using externally generated pulses and internal counters, to prevent extraneous signals from changing any device parameter by accident but, if an unplanned change does occur, there are no means to detect these changes and warn the user. Other methods for fault detection are given in U.S. Pat. No. 5,571,141 wherein a secondary circuit takes over when a specific fault is detected in a primary circuit. That method, however, is very specific to this invention because the secondary circuit is essentially and operationally the same as the primary and thus not generalizable. Similarly, Patent Application Publication No. 20020065540 discloses methods that detect some failures and provide alarms. For example, that patent application discloses the use of two redundant ICs, which must be in agreement for delivery of a drug to occur. Some of the alarm conditions are low battery, empty reservoir, and hardware failure, such as CPU failure using a periodic watch dog timer and over/under delivery of drug. However, that patent application does not disclose or teach how to detect and warn automatically of a device output shutoff or of a device reset, specifically if the fault is not associated with hardware failure. Another, U.S. Pat. No. 6,453,196, discloses methods for detecting abnormal behavior of timing circuits in a medical device by using two different clocks and comparing their pulse counts, but it does not address non-timer-related problems, such as failure of output or accidental reset. All of the above-cited prior art devices fail to operate as programmed when the devices are shut off or are reset accidentally or when the primary output of the devices fail due to external disturbances such as electromagnetic interference, electrostatic interference, radiation or some other unknown reason.
What is needed is a failure detection and warning system having means structured to passively or actively detect faults occurring in a medical device wherein the fault includes an unprogrammed and/or undesired shut off of the medical device or an unprogrammed and/or undesired shut-off of the output of the medical device.